Basic and extendable API for patients

A simple API using django-rest-framework to full-fill #3.

# What's changed:
* Add token authentication
* Add a patient serializer
* Add a patient API viewset
* Add URL endpoints `api` and and `api/patients`
* Add search on patients ex: `api/patients?search=Jane`
* Add filter on patients ex: `api/patients?alive=True`
* Some basic documentation in README.md in `api/`
* Some basic tests

# What's next
* Wait on feedback, add more filters, more search parameters?
Didn't know, in that case the model probably needs to be properly set up to make the uid not required (this just defers to the related Django model definition). For example: mark field as not required, and automatically generate it in `def save`.

If that isn't possible, there's probably a way around that within the API, but it won't be as clean.